cleanup: netx does not use netxlite legacy names (#801)

This diff refactors netx and netxlite to ensure we're not using
netxlite legacy names inside of netx.

To this end, we're cheating a bit. We're exposing a new factory to
get an unwrapped stdlib resolver rather than defining a legacy name
to export the private name of the same factory.

This is actually a fine place to stop, for now, the next and
netxlite refactoring at https://github.com/ooni/probe/issues/2121.

internal/cmd/oohelperd/internal/webconnectivity/webconnectivity_test.go

@@ -54,7 +54,7 @@ func TestWorkingAsIntended(t *testing.T) {
 		Client:            http.DefaultClient,
 		Dialer:            netxlite.NewDialerWithStdlibResolver(model.DiscardLogger),
 		MaxAcceptableBody: 1 << 24,
-		Resolver:          netxlite.NewResolverSystem(),
+		Resolver:          netxlite.NewUnwrappedStdlibResolver(),
 	}
 	srv := httptest.NewServer(handler)
 	defer srv.Close()